1. Field of the Invention
The invention relates to a method for measuring transconductance parameter, and more particularly to a method for measuring transconductance parameter of an oscillating circuit which comprises an inverter.
2. Description of the Related Art
An oscillating circuit with an inverter is one kind of oscillating circuit implemented in integrated circuits. Referring to FIG. 1A, FIG. 1A shows an oscillating circuit 100. The oscillating circuit 100 comprises a feedback resistor Rf and an inverter 110, both coupled between an input terminal OSCI and an output terminal OSCO. FIG. 1B shows a schematic diagram of the oscillating circuit 100 of FIG. 1A, and FIG. 1C shows a small-signal equivalent circuit of the oscillating circuit 100 of FIG. 1A. In FIG. 1B, the inverter 110 comprises a P type metal oxide semiconductor (MOS) transistor 120 and an N type MOS transistor 130, wherein the P type transistor 120 is coupled between a supply voltage VCC and the output terminal OSCO, and the N type transistor 130 is coupled between a ground GND and the output terminal OSCO. In FIG. 1C, a resistor rdsP and a current source gmP×Vgs show a small-signal model of the P type transistor 120, and a resistor rdsN and a current source gmN×Vgs show a small-signal model of the N type transistor 130, wherein gmP is transconductance of the P type transistor 120 and gmN is transconductance of the N type transistor 130.
FIG. 2 shows a conventional measuring circuit for an oscillating circuit 210 with an inverter, wherein the measuring circuit is used to measure transconductance parameter of the oscillating circuit which is referred to as gm. For an oscillating circuit with an inverter, the transconductance parameter may influence oscillating performance. As shown in FIG. 2, an alternating current (AC) voltage source 220 is coupled to an input terminal OSCI of the oscillating circuit 210 via a capacitor C1. Furthermore, a loading resistor RL is coupled to an output terminal OSCO of the oscillating circuit 210 via a capacitor C2. When the AC voltage source 220 provides an AC input voltage Vi to the oscillating circuit 210, an output voltage Vo corresponding to the AC input voltage Vi may be obtained from the loading resistor RL. Therefore, the transconductance parameter of the oscillating circuit 210 can be given by the following equation (1):
                    gm        =                                            Vo              RL                        Vi                    .                                    (        1        )            Moreover, the transconductance parameter of the oscillating circuit 210 is also a sum of the transconductances of the PMOS and NMOS transistors, i.e. gm=gmP+gmN.
However, using an AC measuring method to measure transconductance parameter of an oscillating circuit may increase measuring time and testing cost. Therefore, a DC measuring method for measuring transconductance parameter of an oscillating circuit is desired.